It all depends on which type of sensor kit you buy.
Some have very deep mounting depth required, these are the ones to stay away from as often it means drilling through the bumper Iron also.
The sensors I have used and fitted are quite good.
There are 3 types available.
1. 2 sensor total flush kit, self adjusting also. this sensor is about 22mm diameter 20mm mounting depth and is about 0.6mm thick, so I would almost call this true flush.
2. 4 sensor kit with display. also self adjusting but each sensor has a slight beval so if your bumper has a slight curve you can rotate the sensor so it's horizontal still and pointing the right way, i.e not to the ground. This sensors are not that flush because of the application the are about 1mm thick on say the bottom and then 4 mm thick up the top, this allows for the adjustments of course 22mm mounting depth and 24mm diameter approx.
3. 4 sensor kit true flush like 2 sensor kit, also self adjusting but no display. 22mm diameter. 20mm mounting depth.
All the abouve sensors can be painted to match vehicle colour too and are available from Prolex U.K
Any professional outfit should charge no more then an 2 hours labour to fit, average labour rate at a good car audio/alarm centre is approx £40 per hour.
I hope this helps ...